Cloned MSV proviral DNA and various pieces containing src were transforming in transfection assays. src and one extended terminal redundancy represented the minimal active piece. Tumors were induced by MSV in Japanese quail. These regressed with high frequency and such quail sera were examined for antiviral and anti non-structural protein reactivities. Reversion from MSV transformed state was studied at the proviral DNA and RNA levels in mouse and heterologous species cells.